The present invention relates to adiabatic RF pulses which are used particularly for magnetic resonance imaging (MRI). The invention concerns a new type of adiabatic RF pulse, its uses and a method of creating the same.
Adiabatic pulses are very useful in magnetic resonance imaging (MRI) when precise flip angles are needed, such as in quantitative T1 mapping. But in clinical MRI their use is limited because of the increase in the specific absorption rate (SAR) and the application time to respect the adiabatic condition, compared to sinc or rectangular RF pulses. The present invention concerns a concept based on the sequentialization of adiabatic pulses in order to produce an adiabatic-quality RF pulse. This concept allows to obtain precise flip angles with a smaller SAR and a shorter application time than a true adiabatic pulse, but at the cost of slice-selectiveness.
The new adiabatic RF pulse according to the invention is a combination of two fundamentally opposed characteristics, i.e. precision of the flip angle and power efficiency.
Presently, in order to obtain a precise flip angle for the whole useful volume of MRI antennas, one must use adiabatic type pulses. However, these pulses require a great amount of power. This power is often not available in clinical MRI machines and, could not be used either for reasons of security, since they tend to produce overheating.
In order to avoid difficulties intrinsic to adiabatic pulses, rectangular pulses are often used by MRI clinical machines. They are power efficient and rapid. However, the flip angle produced by rectangular pulses is very sensitive to magnetic homogeneity of the antenna. Therefore, the result error on the flip angle exceeds 50% in an antenna.
This error on the flip angles induces a great variability with respect to the results of quantification sequences based on the hypothesis of a precise flip. The new pulses described herein combine precision for flip and power efficiency. They permit the realisation of very precise quantification measures using a typical clinical MRI machine.
It is in this precision that the new RF concept finds its most important commercial interest. Due to the great uniformity obtained by these pulses, they become an element of choice for quantitative studies such as gel dosimetry.
The dosimetry of analysing radiation by gel is a new field in full emergence. These new developments in dosimetry facilitate validation of radiotherapy treatment and are most likely going to replace old techniques.
Gel dosimetry requires relaxometric measures of great precision on large volumes. The new concept of RF excitation presented herein and its application are perfectly adapted to this technique and surpass existing tools.
T1 cartography by stimulated echo sequences and Look-Locker are the most commonly used to measure relaxation T1 in MRI. They have performances for clinical use needing only a few minutes to produce a map of values of T1. These sequences remain however extremely sensitive to the precision of the RF pulses which re-orient the magnetisation to produce the measured signals.
RF rectangular pulses regularly used in MRI create a flip of the magnetisation directly proportional to the intensity of the magnetic field B1 produced. Clinical antennas have field distributions B1 which fluctuate enormously. As an example, the antenna used to produce images of a head has a field B1 which is distributed in its useful volume in a range going from 0,5 to 1,2 relative to the centre. This spatial variation of B1 systematically creates error on the adjusted values of T1 exceeding 50%.
The development of a new concept RF excitation, following a tangential approach, having adiabatic property permitting the replacement of adiabatic half-pulses and its use as a BIR-4-S2 (B1-Insensitive Rotation-4 AHP-Sequentialized 2 steps) in cartography sequences T1 has permitted to reduce this error to less than 10% in the case of measurements by compensated stimulated echoes and to less than 5% for a Look-Locker.
The BIR-4-S2 has an imprecision on the flip angle of less than 5xc2x0 in a range going from 0,75 to 1,75 about a field of reference B1ref, for a choice of flip of over 360xc2x0. Contrary to adiabatic pulses, it remains a three dimensional RF pulse with low power being able to be used over and over clinically without risking overheating for the patients.
The sequence of compensated stimulated echoes referred to above uses another development: compensation. The signal produced by a traditional sequence tends towards zero following relaxation T1 in time. Sometimes this causes problems when the signal of the last echoes begins to be influenced by noise. The compensation modifies the exponential decrease of the signal. This change operates by taking into account relaxation T1 when calculating the flip angles necessary to the sequence. Compensation transfers a part of the requirement of a volume of signal of the first echoes towards the last. One can then compensate entirely and thus program a sequence which will produce equal signals between all the echoes or compensate partially to maintain the signals of the last echoes all the while conserving a maximum of total signal.
The following description is divided into three chapters, where chapter 1 introduces a new concept for fast 3D adiabatic quality RF pulses for MRI. Chapter 2 describes the TOMROP-Look-Locker T1 mapping using the new BIR-4-S2 RF pulse and Chapter 3 describes a T1 mapping using compensated stimulated echoes and the new BIR-4-S2 RF pulse.
The present invention concerns a system and method for converting an Adiabatic pulse for use in magnetic resonance imaging applications into a pseudo adiabatic pulse.
In accordance with the invention, there is provided, in an MRI system, a method for converting an adiabatic RF pulse into a pseudo adiabatic pulse having at least one step, said MRI system including a magnet for producing a magnetic field about a subject, gradient coils, RF coils, means for emitting an RF pulse and means for receiving RF waves, and means for analyzing said RF waves, said adiabatic RF pulse having at least one adiabatic half passage, said method comprising the steps of:
setting the angle, time step and desired RF power for each time step within a pseudo adiabatic half passage in said means for emitting an RF pulse;
replacing each half passage adiabatic pulse with at least one RF pulse of predetermined power which is adapted to provide a magnetic field vector at said angle from a main magnetic field.
In accordance with the invention, there is also provided an MRI system comprising:
a magnet for producing a magnetic field about a subject;
gradient coils;
RF coils;
means for emitting an RF pulse and means for receiving RF waves;
means for analyzing said RF waves;
means for converting an adiabatic RF pulse having at least one adiabatic half passage into a pseudo adiabatic RF pulse comprising means for setting the angle, time step and desired RF power for each time step within a pseudo adiabatic half passage in said means for emitting an RF pulse; and means for replacing each half passage adiabatic pulse with at least one RF pulse of predetermined power which is adapted to provide a magnetic field vector at said angle from a main magnetic field.